Here is one wacky theory behind the drop at high levels: maybe this is mostly a function of which jobs require certain cognitive levels, and how much those jobs pay.

For example, there are quite a few jobs that pay well which require above average abilities such as management, engineering, medicine, law. But arguably there are jobs that have higher requirements such as theoretical scientists, mathematicians, university professors, philosophers.

Those jobs that have the highest requirements actually pay less than the ones with slightly lower requirements. So maybe what is happening here is that people are more likely to end up with a job that utilizes their unique abilities, but at some point that has and negative impact on outcomes.

For a while I have been on the opinion that we waste the talents of the top 0.1 percent of thinkers by having them debate in the ivory ivory tower while they could be doing a lot more in the real world by going into management, politics, or engineering. But what do I know: maybe 9 dimensional math about strings will prove highly practical some day and add more value than another Da Vinci (who was very involved in practical problems).